The present invention relates to a recording method for magnetooptical disks and apparatus therefor, suitable for performing correct data demodulation by adjusting, at the time of data writing, a timing shift in forming data domains in a buried clock type magneto-optical disk.
Magnetooptical disks have been paid attention as an optical disk capable of changing the written information. A magnetooptical disk uses as its recording film a perpendicular magnetic film on which write data domains are formed through the thermomagnetic effect, and the written data are read through the magnetooptical effect. In data writing, the temperature of the magnetic film is raised to the Curie temperature by the heat of a laser light spot to thereby demagnetize the film. During the course of cooling, an external magnetic field is applied to set a perpendicular magnetization relative to the direction of magnetic field. The optic modulation method and magnetic modulation method are known as the method of forming magnetic domains. According to the former method, an external magnetic field of a predetermined intensity is applied to the perpendicular magnetic film in the direction opposite to the initial magnetization of the film, and the intensity of a laser light pulse is changed with the data to be written. According to the latter method, contrary to the former, the laser light intensity is maintained constant so as to make the temperature of the magnetic film have a temperature higher than or equal to the Curie temperature, and the direction of an external magnetic field is changed with the data to be written.
One of the problems associated particularly with the magnetic modulation method is that the area where the write magnetic field is applied becomes different from the area where the magnetic domain is formed. The reason for this is that the data write or data erase depends on the temperature distribution at the magnetic film and not on the light spot distribution. Although it depends upon the thermal conductivity, linear velocity and the like of a recording film, the peak of temperature distribution generally appears at the back of a light spot position. Since writing data starts at the temperature of the magnetic film higher than the Curie temperature, a domain is formed at a timing delayed from the time when a recording magnetic field is applied.
A data write timing shift, which is a problem associated particularly with the magnetic modulation method, will be described in detail with reference to FIG. 1 which shows the relationship between the overwrite timings and a read-out or reproduced signal. It is assumed not that a light spot 140 is positioned at time t0 within a data recording area of a track 120, i.e., between pits 110 and 111. An addition signal 18 of a P-polarized light component and an S-polarized light component respectively obtained through separation of a reflected light from the perpendicular magnetic film is used for detection of only the signals from the pit train 110 to 112. The addition signal 18 is supplied to a conventional binarizing circuit which binarizes signal 18 by using a threshold value, to obtain a binarized pit signal 25. The pit signal 25 is inputted to a PLL circuit as a phase reference signal to generate clocks 27. The clocks 27 are controlled by the PLL circuit so as to have a predetermined number of clocks between pits. The PLL circuit may employ a circuit arrangement used by a conventional magnetic or optical disk. It is assumed that data 34 modulated by a data modulation circuit is supplied as shown in FIG. 1. The light intensity distribution 141 of the light spot 140 at time t0 is a Gaussian distribution. Since the magnetic domain is generated at the area where the temperature of the perpendicular magnetic film goes higher than or is equal to the Curie temperature, not the light intensity distribution itself but the temperature distribution on the film should be taken into consideration. In practice the temperature distribution at time t0 is subjected to positional shift as shown in FIG. 1, depending on the movement amount of the light spot 140 and the thermal conductivity of the film. In the figure, .DELTA.1 represents a positional shift relative to the center of the light spot where the film temperature goes higher than or equal to the Curie temperature, and .DELTA.2 represents a shift caused by the thermal conductivity and linear velocity of the film. Therefore, a magnetic domain 144 is formed at the position shifted in the direction of disk rotation so that a leading edge portion of a subtraction signal 19 shown in FIG. 1, i.e., a read-out data signal, is shifted by .DELTA.3 from the target leading edge of the modulated magnetic field. On condition that the shift .DELTA.3 takes a constant value, it poses no problem only if the modulated data 34 are shifted, prior to application, by the corresponding amount.
However, in practice, the characteristics of the thermal conductivity and Curie temperature of a recording film are not always uniform. Further, in case where a read/write operation is performed at an equal angular velocity with a constant rotation speed, the linear velocity will change with the radius of a record track to accordingly change the shift amount which also is under the influence of a variation of medium constituents, thus substantially necessiating to check the shift amount for each data write timing (position).
Still further in case where the data modulation method having a self-clocking characteristic, such as Modified Frequency Modulation (MFM) method, Run Length Limit (RLL) method or the like is used, if the data write timing shift is uniform among respective write data domains, a shift among write data patterns poses no special problem. However, in case where the buried clock type disk is used which has pits formed previously to generate therefrom the data read clocks, and particularly where the magnetic modulation method is employed, there is a possibility of a timing shift between write data domains and the clocks during reading the data, in response to which clocks the recording magnetic fields were applied. If the data modulation method without a self-clock characteristic, such as Non Return to Zero (NRZ) method, is employed, the read clock information cannot be obtained directly from the data, thus resulting in an inability of correct demodulation.
If the magnetic modulation method in particular is applied to a buried clock type magnetooptical disk, it is important, as described above, to compensate for the data write timing shift by using proper means. An example of a magnetooptical disk recording apparatus realizing an overwrite operation through the magnetic modulation method is described in Japanese Patent Laid-open Publication JP-A-54-95250. In this apparatus, a laser beam is continuously applied to maintain the recording film at a raised temperature, and a magnet mounted around an objective lens of an optical head is driven in accordance with the data to be written, to thus perform a write and erase operation. This apparatus does not mention the data write timing shift and means for compensating for such shift.